Using a one-step hydrothermal method and at different stirring speeds (Vs), we have synthesized different types of CoFe2O4-based magnetic nanocrystalline samples. With increasing Vs, the sample changes from a nanocomposite of CoFe2O4/Co0.7Fe0.3 (CFO/CF) to single-phase CoFe2O4 (CFO). The maximum magnetization, 88.9 emu/g, and coercivity, 3010 Oe, were obtained when Vs = 0. As Vs increases, the saturation magnetization Ms decreases, because the amount of soft magnetic CF phase decreases. A clear enhancement in the remanence, Mr, was observed, with the maximum Mr/Ms = 0.67; the coercivity Hc also exhibits a local maximum for the sample with Vs = 200 r/min. These trends can be explained well by the interplay between dipolar interaction, magnetocrystalline anisotropy, and shape anisotropy. The stirring speed also influences the impedance of the materials; the related mechanism is discussed.